The present invention relates to a light deflective optical system to be used for exposure during image writing process in an image forming apparatus such as laser beam printers and, more particularly, to a light deflective optical system in which a light beam is reflected twice at a deflective reflecting facet of a rotary-type polygon mirror or the like.
Conventionally, a system comprising at least one deflective reflecting facet, capable of being revolved or swiveled about its rotational axis, and two stationary plane mirrors located to face the deflective reflecting facet has been proposed by Japanese Unexamined Patent Publication S51-6563. In this system, a deflected light beam being reflected at the deflective reflecting facet is successively reflected at the two plane mirrors and is incident again on the deflective reflecting facet so as to be deflected, thereby correcting variations in direction of outputting the deflected light beam which occur due to a tilt of the rotational axis or a tilt of the deflective reflecting facet.
Another system comprising at least one deflective reflecting facet, capable of being revolved or swiveled about its rotational axis, and two stationary plane mirrors located to face the deflective reflecting facet wherein the line of intersection of these plane mirrors extends at a right angle to the rotational axis of the deflective reflecting facet has been proposed by Japanese Unexamined Patent Publication S61-7818 (U.S. Pat. No. 4,796,965). In this system, a light beam is incident on the deflective reflecting facet from a position between the deflective reflecting facet and the two stationary plane mirrors so as to be reflected at the deflective reflecting facet. The reflected light beam is successively reflected at the two plane mirrors and is again incident on and reflected at the deflective reflecting facet. Then, the reflected light beam is outputted through a space between the deflective reflecting facet and the two stationary plane mirrors or between the two stationary plane mirrors, thereby correcting distortion of scan lines.
The system proposed by Japanese Unexamined Patent Publication S51-6563 has such a problem that the deflection angle should be set small in order to prevent interference between the two stationary plane mirrors and an incoming light beam or an outgoing light beam.
On the other hand, the system proposed by Japanese Unexamined Patent Publication S61-7818 is not designed to solve a problem of a case of the deflective reflecting facets being comprised of a rotary-type polygon mirror or the like, that the pitches of scan lines vary due to projecting or sinking of each deflective reflecting facet with regard to the rotational axis of the polygon mirror as a result of, for example, the eccentricity of the polygon mirror about the rotational axis.
In addition, it is not considered to solve a problem that the deflected light beam is twisted at peripheries of scan lines.
Moreover, the system proposed by Japanese Unexamined Patent Publication S61-7818 should employ a high precise plane mirror of which width is extremely narrow for actual use because the stationary plane mirror is interposed between a light beam to be incident on the deflective reflecting facet and a deflected light beam reflected at the deflective reflecting facet. Such high precise plane mirror is difficult to be processed, so this system must be expensive.
The present invention has been made in order to solve the aforementioned problems of the conventional systems. It is an object of the present invention to provide a light deflective optical system, in which variation in deflective light beam in the outgoing direction due to tilt of the rotational axis of the deflective reflecting facets and/or tilt of each deflective reflecting facet is completely corrected, and which can take a wider deflecting angle, eliminate the necessity of use of a plane mirror of which width is narrow, prevent variation in pitch of scan lines even with projecting or sinking of the deflective reflecting facet, and further prevent a deflective light beam from twisting.
A light deflective optical system achieving the aforementioned object comprises at least one deflective reflecting facet, parallel to a rotational axis and capable of being revolved or swiveled about said rotational axis, and two stationary plane mirrors, disposed to face the deflective reflecting facet, wherein a light beam incident on and thus reflected at said deflective reflecting facet is reflected successively at said two stationary plane mirrors and this reflected light beam is again incident on and reflected at said deflective reflecting facet, and is characterized in that
said two stationary plane mirrors are disposed perpendicularly to an incident plane to have a space therebetween on the condition that said incident plane is defined as a face containing a light beam to be first incident on said deflective reflecting facet and being parallel to said rotational axis, and
the light beam to be first incident on said deflective reflecting facet passes through the space between said two stationary plane mirrors and the deflected light beam after second reflection at said deflective reflecting facet outputs through the space between said two stationary plane mirrors.
In this case, it is preferable that the following condition is satisfied:
0.92xe2x89xa6(xcex82/xcex81)xe2x89xa61.25xe2x80x83xe2x80x83(30) 
wherein xcex81 indicates the incidence angle of the light beam to be first incident on the deflective reflecting facet relative to the deflected reflecting facet under conditions that the incident plane contains the deflected light beam of the first reflection and xcex82 indicates the outgoing angle of the deflected light beam of the second reflection at the deflective reflecting facet under conditions that the incident plane contains the deflected light beam of the second reflection.
In addition, it is preferable that the following condition is satisfied:
0.1xc2x7|xcex2xe2x80x2xc2x7(sin xcex81xe2x88x92sin xcex82)|xe2x89xa60.25xc2x7LPxe2x80x83xe2x80x83(35) 
wherein assuming the shifting amount of the outgoing light beam in a section containing said rotational axis as Y which is generated due to projecting or sinking of said deflective reflecting facet with respect to said rotational axis and assuming the shifting amount of the scanning point on a scanned surface corresponding to the shifting amount Y as Yxe2x80x2 which is generated due to an optical system between the second reflection point on said deflective reflecting facet and the scanned surface, xcex2xe2x80x2 is defined as xcex2xe2x80x2=Yxe2x80x2/Y and LP is the pitch of scan lines on said scanned surface, with dimensions being in millimeters.
In this case, the following condition is further preferably satisfied:
0.1xc2x7|xcex2xe2x80x2xc2x7(sin xcex81xe2x88x92sin xcex82)|xe2x89xa60.125xc2x7LPxe2x80x83xe2x80x83(36) 
with dimensions being in millimeters.
The image forming apparatus is preferably characterized by satisfying the condition of xcex81:xcex82≈1:1.
For example, the light deflective optical system of the present invention may be used for exposure during image writing process of an image forming apparatus.
According to the present invention, a light deflective optical system can be provided, in which variation in deflective light beam in the outgoing direction due to tilt of the rotational axis of the deflective reflecting facets and/or tilt of each deflective reflecting facet is completely corrected, and which can take wider deflecting angle, employ relatively wide plane mirrors can be employed as the stationary plane mirrors. That is, cheaper high precise plane mirrors which are easy to be processed can be employed as the stationary plane mirrors. In addition, the light deflective optical system can prevent variation in pitch of scan lines even with projecting or sinking of the deflective reflecting facet and prevent the outgoing light beam from twisting.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The invention accordingly comprises the features of construction, combinations of elements, and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.